PROGRESS OF LIFE. 595 



through ages of unceasing change, declare emphatically the unity of 

 system in Nature. 



(2.) This truth is further manifested, in the fact of a general paral- 

 lelism between the progress of the earth's life and the successive phases 

 in embryonic development. The almost egg-like simplicity of the earli- 

 est living species of the rocks, — the Rhizopods among animals, and 

 the Infusorial jitlants, — is the first illustration Geology presents. An 

 animal without limbs, without any sense beyond the general sense of 

 feeling, without a circulating system, without even a stomach, except 

 such as it may extemporize when needed, and with the work of diges- 

 tion, respiration, and reproduction performed by the same protoplasmic 

 material that makes up the mass of the body of the infinitesimal 

 Rhizopod, is, as to complexity of organization, but little removed from 

 a germ ; and such, we have reason to believe, was the beginning of 

 the system of animal life. 



Again, we find some of the earliest Crustaceans of the Phyllopod 

 group closely resembling the young of some of the higher groups 

 of living Crustaceans ; and the early Fishes having cartilaginous skele- 

 tons, just as is now true of the higher Vertebrates when in the em- 

 bryonic condition. 



Again, the Gars of the present day have a vertebrated lobe to 

 the tail, which they lose on becoming adults ; and so the Gars had 

 vertebrated tails in the young world, that is in Paleozoic time, which 

 feature was lost in the progress of the Mesozoic era. The Amphib- 

 ians afford a very similar illustration. So also the Birds ; for, as the 

 young often have a tail of several disconnected vertebras, which con- 

 tracts much on passing to the adult stage, so the earliest known of the 

 Bird type had long, vertebrated tails, such as no modern Bird can boast 

 or complain of. 



Among the modern free Crinoids (Comatulids), the young, for a 

 while, live attached to some support ; and so, in the young world, the 

 adult Crinoids had pedicels, and were attached species. In the ex- 

 isting Echini, as observed by A. Agassiz, the number of vertical series 

 of plates in the shell of the young is often more than the adult num- 

 ber, twenty, and the adult shows this excess in the plates right around 

 the mouth, the plates there being those of the young ; and so, in the 

 Echini of the young or Paleozoic world, the adults had an excessive 

 number of series of plates, while later they have only the normal twenty. 



(3.) The system of progress was a system of successive specializations ; 

 and in this it was parallel in idea with embryonic development; for, 

 while in the earliest species all the functions were performed by one 

 and the same protoplasmic mass, as the grade of species rose, these 

 functions, one after another, had special organs to carry them forward. 



